You couldn't just argue that they are in apposition with each other, implying that cold and death are the same thing? (My Latin dictionary actually lists the "Cold of Death" as one of the common meanings of Frigus, right after "Cold Place")
Or, more reasonably, maybe Patrian and/or its descendants are just a bit different that Latin. There are a few other divergences in the game, pretty much any of which could be explained by whoever wrote it having a Latin dictionary but not understanding how words need to be declined. The first to come to mind is the order of Grigori medics being named the Ordine Medicos (when it probably should be Ordo Medicorum, but the ablative and accusative were used in place of the nominative and genitive), and the second is the city of Tentatio-onis (which looks like someone looked up temptation/trial and didn't recognize the part after the hyphen is just the genitive case ending). Perhaps Letum Frigus is supposed to be Letum Frigoris (the Death of Cold, referring to Mulcarn's demise there) or Letum Frigore (Death by Cold, referring to the fate of their enemies)